Since you want Patrick and Katie (Mack) Callahan, I think the Thomas born on August 18, 1881 is not in your family. The Thomas who died in 1906 in Ashuelot is probably the correct Thomas to be a sibling of your grandfather.

Since this Ashuelot family seems to have a family plot at St. Michael's Cemetery in Brattleboro, you might want to check with them to see if Patrick and Katie Mack are buried there.

Also the "son" Martin Callahan listed with Patrick in the 1900 census is probably a census taker's error. I think it's probably actually "brother-in-law" Martin Murphy. Martin Murphy turns out to be the same age and also immigrated in 1893.

One final thing - and I hope I'm not crazily overwhelming you; I tend to do that - but I found a report about Thomas Callahan's death so here that is:DEATH OF THOMAS CALLAHAN. State Of New Hampshire. In Board Of Railroad Commissioners.

Concord, N. H., December 24, 1906.Investigation at Concord, December 21, 1906.Witnesses: Edwin A. Pratt, engineer, East Northfield, Mass.; Edward J. Madden, West Northfield, Mass.About 6:05 o'clock on the evening of November 22, 1906, train No. 65 on the Ashuelot branch of the Boston & Maine Railroad, when approaching the station at West Swanzey, run over and fatally injured Thomas Callahan, a young man about thirty-two years of age. The point where the accident occurred was distant from the station in a southerly direction about three hundred (300) feet and was also about the same distance from a highway crossing. The train which caused the fatality was making about schedule time, running from thirty to thirty-five miles an hour; the engineer had sounded the usual crossing whistle, and the bell was ringing when the engineer observed a dark object upon the track, which he at first believed to be a shadow cast from an electric light, which was located a short distance away. So near was the engine to the object when it was first noticed that to make a stop before hitting it was an absolute impossibility. The man, for such the object proved to be, was lying down lengthwise upon the east rail. Upon arrival at the station the engineer immediately informed the trainmen that he had run over something. An investigation was at once made and the lifeless body of Thomas Callahan was found. Death must have been instantaneous. It appeared from the evidence before us that this man had been in Keene that day and had been placed under arrest for some misdemeanor. The complainant failing to appear against him, he was released from custody upon condition that he leave the city. While in Keene he had been drinking, but was not so intoxicated that the city marshal thought it necessary to detain him on that account. He hired a conveyance to take him to West Swanzey and during the afternoon was at Whitcomb's shop, where he met several acquaintances among the employees. It was here observed that he had been drinking quite heavily. His home was at Ashuelot, distant about ten miles, and it was suggested that he had started down the track with the purpose of walking home. While it is entirely a matter of conjecture, it is reasonable to conclude that he was overcome by the effects of the liquor he had drunk, fell down upon the track and was run over and killed in the manner described. There was no evidence submitted to sustain the theory of suicide. We cannot find that the trainmen failed to handle the train properly or omitted to do anything which could reasonably be required of them. Mr. Callahan was a trespasser, using the railroad track as a highway, for which there was no necessity, and must be regarded as a victim of his own error.GEORGE E. BALES, For the Board